1. Field of the Invention (Technical Field)
The present invention relates to methods and systems for cleaning fouled recuperators in microturbine power systems.
2. Background
Microturbines are high speed, single or dual shaft, low compression turbine-driven generators of electricity and/or other power, and producing up to about 500 kW. They are multi-fuel, modular distributed power generation units having multiple applications. They offer the capability to produce electricity at a lower cost per kilowatt than do central plants, and they do not require the installation of expensive infrastructure to deliver power to the end users. Thus, in parts of the world lacking the transmission and distribution lines of a basic electric infrastructure, commercialization of microturbines may be greatly expedited. In the United States and other countries already having a suitable electric infrastructure, distributed generation units will allow consumers of electricity to choose the most cost-effective method of electric service. In addition to primary power generation, microturbines also offer an efficient way to supply back-up power, uninterruptible power, peak-shaving, and combined heat (or chilling) and power through the use of cogeneration equipment. Other applications for microturbines exist as well.
In a typical microturbine system, compressed air is directed to a recuperator in which the temperature of the compressed air is increased using waste heat from the turbine exhaust gas. After being heated in the recuperator, the compressed air is directed to a combustor in which additional heat is added to further raise the temperature of the compressed air.
The recuperator in microturbine power systems frequently foul during operation. Fouling is the deposition of the unburnt hydrocarbons on the recuperator heat transfer surface, typically on the cooler portion of the exhaust gas side of the recuperator heat transfer surface downstream of the combustor. When the latter portion of the recuperator is subject to such carbon deposits, the recuperator effectiveness decreases significantly, which in turn decreases the overall microturbine system efficiency and increases fuel usage.
Existing methods for preventing and/or resolving fouling include the following:
U.S. Pat. No. 4,047,972, entitled Method for Thermally De-Sooting Heat Transfer Surfaces, to Stumbar et al., issued Sep. 13, 1977, discloses utilizing a CO monitor to control thermal de-sooting of a heat exchanger in order to prevent run-away reactions. This patent does not teach or disclose a recuperator disposed so as to pre-heat fuel utilizing exhaust gases. The patent does disclose (col. 3, lines 35-63) feeding nitrogen or another inert gas into the heat exchanger tubes (to prevent corrosion) and increasing the temperature of the exhaust gases from the gas turbine through the heat exchanger until the CO level, as measured by a continuous monitoring device, is within acceptable levels. This thus oxidizes and removes carbonaceous deposits within the heat exchanger. The patent does not teach use of temperature differential of heat exchange to determine when fouling exceeds acceptable limits.
U.S. Pat. No. 4,115,998, entitled Combustion Monitor, to Gilbert et al., issued Sep. 26, 1978, discloses a combustion monitoring system with a plurality of sensors. The gas turbine disclosed incorporates a compressor, combustor and turbine, but does not include a recuperator. The sensors include temperature sensors for ascertaining xe2x80x9cfailure modesxe2x80x9d, but the patent does not disclose using temperature sensors for ascertaining fouling or deposition of unburnt hydrocarbons.
U.S. Pat. No. 4,869,209, entitled Soot Chaser, to Young, issued Sept. 26, 1989, discloses an automatic programmable cleaning system for heat transfer tubes in an engine exhaust gas waste heat recovery boiler. This patent is not directed to turbine systems as such, but is rather directed to engine exhaust gas waste heat recovery boilers. The cleaning method involves release of water utilizing an automatic programmable cleaning system, which water vaporizes and exhausts carbon through the waste heat exhaust gas stream. No recuperator is disclosed, and no temperature sensors are disclosed.
U.S. Pat. No. 5,794,446, entitled Power Plant Performance Management Systems and Methods, to Earley et al, issued Aug. 18, 1998, discloses a temperature-based system for use in monitoring a steam powered electric power generating system. This system employs a multi-turbine generator system, with a boiler, superheater and reheater, but not a recuperator. The temperature detectors are utilized to determine the presence of condensate or feed water in the system, thereby optimizing performance. The temperature detectors and computer compare first temperatures with second temperatures, and can evaluate the temperatures in relation to one another, and compares the difference to a standard temperature difference.
While the general principal of heating a device through which a carbon-based fuel passes to a xe2x80x9cself-cleaningxe2x80x9d temperature, to combust unburnt hydrocarbons, is known, the present invention provides three novel means for generating the proper temperatures: (1) blocking the exhaust to reduce heat loss and thereby increase heat transfer to the recuperator; (2) inducing an air flow with an air pump to convection heat the recuperator core from the residual heat in the combustor, engine core and hot parts of the recuperator; and (3) use of an electric heat probe or gas torch inserted into the recuperator inlet plenum.
Furthermore, in contrast with known systems and methods, the invention monitors the difference between the recuperator hot gas in and recuperator air out to determine fouling; when this difference is determined to be above a specified limit, then the recuperator is fouled. This control mechanism is then utilized to activate one or a combination of the disclosed xe2x80x9cself-cleaningxe2x80x9d mechanisms.
While various temperature analysis systems are known in the art, there is no disclosure of a temperature analysis system measuring the difference between turbine exhaust gas temperature in and air temperature out to determine fouling. Further, there is no disclosure of a system utilizing this or any analogous control parameter to initiate a xe2x80x9cself-cleaningxe2x80x9d mechanism, including, for example, the disclosed xe2x80x9cself-cleaningxe2x80x9d mechanisms.
The present invention is of a microturbine power system and of a method of operating same comprising automatically detecting fouling of a recuperator and automatically cleaning the recuperator. In the preferred embodiment, detecting comprises determining a temperature difference between air exiting from the recuperator and gas entering the recuperator, preferably via a recuperator gas inlet temperature sensor and a recuperator air outlet temperature sensor, and preferably employing an engine control unit receiving input from the sensors. Cleaning preferably comprises initiating cleaning via an engine control unit when fouling is detected. Cleaning can employ a recuperator gas exhaust block, gas heating (preferably proximate a recuperator inlet plenum and employing an electric heat probe or a gas torch), and/or an air pump to convection heat a core of the recuperator.
Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.